Structure and method for contact pads having a recessed bondable metal plug over of copper-metallized integrated circuits

ABSTRACT

A metal structure for an integrated circuit, which has copper interconnecting metallization ( 311 ) protected by an overcoat layer ( 320 ). A portion of the metallization is exposed in a window ( 301 ) opened through the thickness of the overcoat layer. The metal structure comprises a patterned conductive barrier layer ( 330 ) positioned on the copper metallization, wherein this barrier layer forms a trough with walls ( 331 ) conformal with the overcoat window. The height ( 331   a ) of the wall is less (between 3 and 20%) than the overcoat thickness ( 320   a ), forming a step ( 340 ). A plug ( 350 ) of bondable metal, preferably aluminum, is positioned in the trough and has a thickness equal to the trough wall height ( 331   a ).

FIELD OF THE INVENTION

The present invention is related in general to the field of electronicsystems and semiconductor devices and more specifically to bond padstructures and fabrication methods of copper metallized integratedcircuits.

DESCRIPTION OF THE RELATED ART

In integrated circuits (IC) technology, pure or doped aluminum has beenthe metallization of choice for interconnection and bond pads for morethan four decades. Main advantages of aluminum include easy ofdeposition and patterning. Further, the technology of bonding wires madeof gold, copper, or aluminum to the aluminum bond pads has beendeveloped to a high level of automation, miniaturization, andreliability.

In the continuing trend to miniaturize the ICs, the RC time constant ofthe interconnection between active circuit elements increasinglydominates the achievable IC speed-power product. Consequently, therelatively high resistivity of the interconnecting aluminum now appearsinferior to the lower resistivity of metals such as copper. Further, thepronounced sensitivity of aluminum to electromigration is becoming aserious obstacle. Consequently, there is now a strong drive in thesemiconductor industry to employ copper as the preferred interconnectingmetal, based on its higher electrical conductivity and lowerelectromigration sensitivity. From the standpoint of the mature aluminuminterconnection technology, however, this shift to copper is asignificant technological challenge.

Copper has to be shielded from diffusing into the silicon base materialof the ICs in order to protect the circuits from the carrier lifetimekilling characteristic of copper atoms positioned in the siliconlattice. For bond pads made of copper, the formation of thincopper(I)oxide films during the manufacturing process flow has to beprevented, since these films severely inhibit reliable attachment ofbonding wires, especially for conventional gold-wire ball bonding. Incontrast to aluminum oxide films overlying metallic aluminum, copperoxide films overlying metallic copper cannot easily be broken by acombination of thermocompression and ultrasonic energy applied in thebonding process. As further difficulty, bare copper bond pads aresusceptible to corrosion.

In order to overcome these problems, the semiconductor industry adopteda structure to cap the clean copper bond pad with a layer of aluminumand thus re-construct the traditional situation of an aluminum pad to bebonded by conventional gold-wire ball bonding. The described approach,however, has several shortcomings. First, the fabrication cost of thealuminum cap is higher than desired, since the process requiresadditional steps for depositing metal, patterning, etching, andcleaning. Second, the cap must be thick enough to allow reliable wirebonding and to prevent copper from diffusing through the cap metal andpossibly poisoning the IC transistors.

Third, the aluminum used for the cap is soft and thus gets severelydamaged by the markings of the multiprobe contacts in electricaltesting. This damage, in turn, becomes so dominant in the everdecreasing size of the bond pads that the subsequent ball bondattachment is no longer reliable. Finally, the elevated height of thealuminum layer over the surrounding overcoat plane enhances the risk ofmetal scratches and smears. At the tight bond pad pitch of many highinput/output circuits, any aluminum smear represents an unacceptablerisk of shorts between neighbor pads.

SUMMARY OF THE INVENTION

A need has therefore arisen for a metallurgical bond pad structuresuitable for ICs having copper interconnection metallization whichcombines a low-cost method of fabricating the bond pad structure, aperfect control of up-diffusion, a risk elimination of smearing orscratching, and a reliable method of bonding wires to these pads. Thebond pad structure should be flexible enough to be applied for differentIC product families and a wide spectrum of design and processvariations. Preferably, these innovations should be accomplished whileshortening production cycle time and increasing throughput, and withoutthe need of expensive additional manufacturing equipment.

One embodiment of the invention is a metal structure for an integratedcircuit, which has copper interconnecting metallization protected by anovercoat layer. A portion of the metallization is exposed in a windowopened through the thickness of the overcoat layer. The metal structurecomprises a patterned conductive barrier layer positioned on the coppermetallization, wherein this barrier layer forms a trough with wallsconformal with the overcoat window. The height of the wall is preferablyless (between 3 and 20%) than the overcoat thickness. A plug of bondablemetal, preferably aluminum, is positioned in the trough and has athickness substantially equal to the trough wall height; the surface maybe flat with the walls.

Another embodiment of the invention is a wafer-level method offabricating a metal structure for a contact pad of an integratedcircuit, which has copper interconnecting metallization protected by anovercoat layer. The method comprises the steps of opening a window inthe overcoat layer to expose the copper metallization, whereby thewindow has walls reaching through the thickness of the overcoat layer. Abarrier metal layer is then deposited over the wafer to cover theexposed copper metallization, the window walls, and the overcoatsurface. Next, a bondable metal layer (preferably aluminum) is depositedover the barrier layer in a thickness sufficient to fill the overcoatwindow. Next, the wafer is chemically-mechanically polished so that thelayers of bondable metal and barrier metal are removed over theovercoat, while these layers remain inside the window. The continuedchemical-mechanical polishing step is controlled (polishing speed, time,and temperature) so that a pre-determined amount of metal height(between 3 and 20%) is removed from the filled window, whereby astructural step is formed from the overcoat surface to the remainingmetal.

Embodiments of the present invention are related to wire-bonded ICassemblies, semiconductor device packages, surface mount and chip-scalepackages. It is a technical advantage that the invention offers alow-cost method of reducing the risk of aluminum-smearing or -scratchingand electrical shorting between contact pads. The assembly yield of highinput/output devices can thus be significantly improved. It is anadditional technical advantage that the invention facilitates theshrinking of the pitch of chip contact pads without the risk of yieldloss due to electrical shorting. Further technical advantages includethe opportunity to scale the assembly to smaller dimensions, supportingthe ongoing trend of IC miniaturization.

The technical advantages represented by certain embodiments of theinvention will become apparent from the following description of thepreferred embodiments of the invention, when considered in conjunctionwith the accompanying drawings and the novel features set forth in theappended claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 depicts a schematic cross section of a contact pad of anintegrated circuit (IC) with copper metallization according to knowntechnology. The bondable metal is added as an additional layer elevatedover the wafer surface.

FIG. 2 illustrates a schematic cross section of two wire-bonded contactpads of a copper-metallized IC in known technology. The elevatedbondable metal layers have been scratched and smeared, causing anelectrical short.

FIG. 3 is a schematic cross section of an embodiment of the inventiondepicting a contact pad of an IC with copper metallization, wherein thecontact pad has a bondable metal plug.

FIG. 4 is a schematic cross section of the bond pad metallizationaccording to the invention, with a ball bond attached to the bondablemetal plug.

FIG. 5 is a block diagram of the device fabrication process flowaccording to another embodiment of the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The technical advantages offered by the invention can be bestappreciated by comparing an embodiment of the invention with theconventional method of wire-bonding a contact pad of an integratedcircuit (IC) chip, which uses copper as interconnecting metal. Anexample of a conventional structure is depicted in FIG. 1. In theschematic cross section of an IC contact pad generally designated 100,101 is an intra-level dielectric, which may consist of silicon dioxide,a low-k dielectric, or any other suitable insulator customarily used inICs. 102 represents the top level IC copper metallization (thicknesstypically between 200 and 500 nm, contained by barrier layers 103 a and103 b (typically tantalum nitride, typically 10 to 30 nm thick) fromdiffusing into other IC materials. In the essentiallymoisture-impermeable overcoat layer 104 (typically between 500 to 1000nm of silicon nitride, silicon oxynitride, or silicon dioxide,single-layered or multi-layered) is contact window 110, usually between40 to 70 μm wide, which exposed the copper metallization 102 forestablishing a contact. Barrier layer 103 b overlaps overcoat 104 aroundthe window perimeter to create a metallization width 111, which is thuslarger than window 110 (typically about 45 to 75 μm diameter). The samewidth 111 holds for the bondable metal layer 120, which is aluminum or acopper-aluminum alloy. For reliable wire bonding, layer 120 hastypically a thickness 121 between 700 and 1000 nm.

This considerable height 121 of the patterned aluminum layer 120represents a substantial risk for accidental scratching or smearing ofthe aluminum. There are numerous wafer and chip handling steps in atypical assembly process flow after the aluminum patterning. The mostimportant steps include back-grinding; transporting the wafer from thefab to the assembly facility; placing the wafer on a tape for sawing;sawing and rinsing the wafer; attaching each chip onto a leadframe; wirebonding; and encapsulating the bonded chip in molding compound. At eachone of these process steps, and between the process steps, accidentalscratching or smearing could happen.

An example is schematically indicated in FIG. 2, which is a crosssection through two bonding pads 201 and 202 in close proximity(distance 230). The aluminum layer 210 of pad 201 and the aluminum layer220 of pad 202 have been scratched so that the aluminum is smearedtogether at 240. As a consequence, the pads of bonds 250 and 251 form anelectrical short.

An embodiment of the invention is shown in FIG. 3, illustrating aschematic cross section of a portion 300 of a semiconductor wafer. Theinterlevel insulating material 310, made, for instance, of low-kdielectric material, silicon dioxide, or a stack of dielectricmaterials, is covered by a protective overcoat 320. Preferred overcoatmaterials are practically moisture impermeable or moisture retaining,and mechanically hard; examples include one or more layers of siliconnitride, silicon oxynitride, silicon carbide, or a stack of insulatingmaterials including polyimide. The overcoat has a thickness 320 a in therange from 0.5 to 1.5 μm, preferably 1.0 μm. Windows 301 and 302 in theovercoat are opened to reach the top layer of the interconnectingmetallization. The top metal layer consists of copper or a copper alloyand has a thickness preferably in the range from 0.2 to 0.5 μm. Layerportion 311 in window 301 serves the bond pad, layer portion 312 inwindow 302 is the metal in the scribe street. The copper metallizationis contained by barrier layer 313 a, and 131 b respectively, fromdiffusing into insulator 310 or other integrated circuit materials;barrier layers 313 a and 313 b are preferably made of tantalum nitrideand about 10 to 30 nm thick.

In order to establish low-resistance ohmic contact to the copper, oneore more conductive barrier layers 330 are deposited over the copper, asindicated in FIG. 3. For a single layer, tantalum nitride is preferablyselected. For a couple of layers, the first barrier layer is preferablyselected from titanium, tantalum, tungsten, and alloys thereof; thelayer is deposited over the exposed copper 311 with the intent toestablish good ohmic contact to the copper by “gettering” the oxide awayfrom the copper. A second barrier layer, commonly nickel vanadium, isdeposited to prevent outdiffusion of copper. The barrier layer has athickness preferably in the range from 0.02 to 0.03 μm. In the windows,the layers form a trough with walls conformal with the respectiveovercoat window. In FIG. 3, the trough walls for the bond pad window aredesignated 331, and the walls for the scribe street window 332.

As FIG. 3 indicates, the wall height 331 a and 332 a, respectively, isless than the overcoat thickness 320 a. In the preferred embodiment, thewall height 331 a (and 332 a) is between about 6 and 30% less than theovercoat thickness 320 a. Consequently, a step height 340 of about 0.1to 0.2 μm between the overcoat surface 320 b and the barrier wallsurface 331 b (and 332 b) is created.

The volume enclosed by the barrier layer trough is filled with a plug ofbondable metal, which has a thickness substantially equal to the troughheight. The bondable metal is preferably aluminum or an aluminum alloy,such as aluminum-copper alloy. In FIG. 3, the through formed by barrierlayer 330 with wall 331 is filled by plug 350, and the trough formed bybarrier layer 330 with wall 332 is filled by plug 351. Since the plugthickness is about equal to the trough wall height (331 a and 332 a,respectively), it exhibits the same recess step 340 relative to theovercoat surface 320 b. Consequently, the plug 350, and 351respectively, is protected against accidental scratches of the overcoatsurface 320 b, providing the undisturbed plug metal for reliable ballbonding.

The cross section of FIG. 4 illustrates schematically the contact pad ofFIG. 3 after the chip has been singulated from the wafer in a sawingprocess (scribe street indicated by 410) and a ball bond has beenattached. A free air ball 401 (preferably gold) of a metal wire 402(preferably gold) is pressure-bonded to the undisturbed surface 403 a ofthe plug 403 (preferably aluminum or an aluminum alloy). In the bondingprocess, intermetallic compounds 404 are formed in the contact region ofball and plug.

Another embodiment of the invention is a wafer-level method offabricating a metal structure for a contact pad of an integratedcircuit, which has copper interconnecting metallization. The wafer isprotected by an overcoat layer, which includes silicon nitride as apractically moisture-impermeable material. The process flow is displayedin the schematic block diagram of FIG. 5. The method, starting at step501, opens at a window in the overcoat layer at step 502 in order toexpose the copper metallization. The window has walls reaching throughthe thickness of the overcoat layer.

In the next process step 503, a barrier metal layer is deposited overthe wafer. Preferred barrier metal choices include tantalum or tantalumnitride, and nickel vanadium. Inside the window, this conductive barriermetal layer covers the exposed copper metallization and the windowwalls; outside the window, the barrier layer covers the overcoatsurface. In step 504, a bondable metal layer is deposited over thebarrier layer in a thickness sufficient to fill the overcoat window.Preferred bondable metal choices include aluminum and aluminum alloy.

In the next process steps, the wafer is subjected to achemical-mechanical polishing step. This process uses commerciallyavailable equipment, for instance the chemical-mechanical polishing(CPM) system by Applied Material Mirra Mesa, U.S.A., which is a MIPSS(missed-in-place slurry system) and has built-in control capabilities.While one polishing step may be sufficient, the block diagram of FIG. 5illustrates two subsequent polishing steps. In step 505, a coarsepolishing powder is used to polish the wafer so that the layer ofbondable metal and the barrier metal layer are removed over theovercoat, while these layers remain inside the window. An example of asuitable coarse powder is the slurry SS-12 of the Cabot Corporation,U.S.A. The removal rate in step 505 is approximately 400 nm/min.

In step 506, a fine polishing powder is used to chemically-mechanicallyfine-polish the wafer under careful control of rotation speed, polishingtime, and specimen temperature. The removal rate in step 506 isapproximately 100 nm/min. A suitable polishing equipment is again theApplied Material Mirra Mesa CPM system. For the fine polishing step 506,this system is operated with Ceria HSS, a highly selective slurry madeby Hitachi Corporation, Japan.

Alternatively, the coarse polishing step may be followed by a controlledchemical etching step.

Under continued control of speed, time and temperature, thechemical-mechanical fine polishing step 507 (or, alternatively, thechemical etching step) selectively affects the metal in the window(which is relatively soft compared to the hard overcoat) so that apre-determined amount of metal height is selectively removed from thefilled window. Consequently, a structural step is formed from theovercoat surface to the remaining metal in the window. This stepcomprises between 3 and 20% of the overcoat thickness. For an overcoatwith 800 nm thickness, the step is preferably about 80 nm; for anovercoat with 1000 nm thickness, the step is preferably about 150 nm.The method concludes at step 508.

While this invention has been described in reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription.

As an example, the fabrication method can be modified so that thechemical-mechanical polishing is performed using a single slurry, butunder modified conditions of polishing rotation and time, stillproducing the desired height step between the overcoat and the metalplug. As another example, the bondable metal plug has a surface on aflat level with the trough walls of the barrier layer.

It is therefore intended that the appended claims encompass any suchmodifications and embodiments.

1. A metal structure for an integrated circuit having copperinterconnecting metallization protected by an overcoat layer, portionsof said metallization exposed in a window opened through the thicknessof said overcoat layer, comprising: a patterned conductive barrier layerpositioned on said copper metallization in said window, said barrierlayer forming a trough having walls conformal with said window and atrough height less than said overcoat thickness; and a plug of bondablemetal positioned in said trough, said plug having a thicknesssubstantially equal to said trough height so that said window is a padsuitable for wire bonding.
 2. The metal structure according to claim 1wherein said overcoat thickness ranges from about 0.6 to 1.5 μm.
 3. Themetal structure according to claim 1 wherein said overcoat comprises oneor more layers of silicon nitride, silicon oxy-nitride, silicon dioxide,silicon carbide, or other moisture-retaining compounds.
 4. The metalstructure according to claim 1 wherein said wall height is between 6 and30% less than said overcoat thickness, creating a step height of 0.1 to0.2 μm.
 5. The metal structure according to claim 1 wherein saidbondable metal is aluminum or an aluminum alloy.
 6. The metal structureaccording to claim 1 wherein said plug has a thickness between about 0.4and 1.4 μm.
 7. The metal structure according to claim 1 wherein saidplug has a surface on a flat level with said trough walls.
 8. The metalstructure according to claim 1 further comprising a ball bond attachedto said plug.
 9. The metal structure according to claim 1 wherein saidbarrier layer comprises tantalum nitride.
 10. The metal structureaccording to claim 1 wherein said barrier layer is selected from a groupconsisting of tantalum, titanium, tungsten, molybdenum, chromium,vanadium, alloys thereof, stacks thereof, and chemical compoundsthereof.
 11. The metal structure according to claim 1 wherein saidbarrier layer has a thickness between about 0.02 and 0.03 μm.
 12. Ametal structure for an integrated circuit having copper interconnectingmetallization protected by an overcoat layer, portions of saidmetallization exposed in a window opened through the thickness of saidovercoat layer, comprising: a patterned conductive barrier layerpositioned on said copper metallization in said window, said barrierlayer forming a trough having walls conformal with said window and atrough height substantially equal to said overcoat thickness; and a plugof bondable metal positioned in said trough, said plug having athickness substantially equal to said trough height so that said windowis a pad suitable for wire bonding.
 13. A wafer-level method offabricating a metal structure for a contact pad of an integrated circuithaving copper interconnecting metallization protected by an overcoatlayer including silicon nitride, comprising the steps of: opening awindow in said overcoat layer to expose said copper metallization, saidwindow having walls reaching through the thickness of said overcoatlayer; depositing a barrier metal layer over said wafer to cover saidexposed copper metallization, window walls, and overcoat surface;depositing a bondable metal layer over said barrier layer in a thicknesssufficient to fill said overcoat window; and chemically-mechanicallypolishing said wafer so that said layers of bondable metal and barriermetal are removed over said overcoat outside said window.
 14. The methodaccording to claim 13 further comprising the step of controlling thecontinued chemical-mechanical polishing step so that a pre-determinedamount of metal height is selectively removed from said filled window,whereby a structural step is formed from said overcoat surface to theremaining metal.
 15. The method according to claim 13 wherein said stepof chemically-mechanically polishing comprises a step of coarsepolishing followed by a step of fine polishing.
 16. The method accordingto claim 15 wherein said step of chemically-mechanically coarsepolishing comprises a removal rate of approximately 400 nm/min.
 17. Themethod according to claim 15 wherein said step ofchemically-mechanically fine polishing is selective and comprises aremoval rate of approximately 100 nm/min.
 18. The method according toclaim 13 wherein said step of chemically-mechanically polishingcomprises a step of coarse polishing followed by a step of etching. 19.The method according to claim 13 wherein said controls include polishingspeed, time, and temperature.
 20. The method according to claim 13wherein said step comprises between 3 and 20% of said overcoatthickness.